Cystathionine enzymic synthesis

Homocysteine (Hey) metabolism is closely linked to that of the essential amino acid methionine and thus plays a central role in several vital biological processes. Methionine itself is needed for protein synthesis and donates methyl groups for the synthesis of a broad range of vital methylated compounds. It is also a main source of sulphur and acts as the precursor for several other sulphur-containing amino acids such as cystathionine, cysteine and taurine. In addition, it donates the carbon skeleton for polyamine synthesis [1,2]. Hey is also important in the metabolism of folate and in the breakdown of choline. Hey levels are determined by its synthesis from methionine, which involves several enzymes, its remethylation to methionine and its breakdown by trans-sulphuration. 

Cysteine is formed in plants and in bacteria from sulfide and serine after the latter has been acetylated by transfer of an acetyl group from acetyl-CoA (Fig. 24-25, step f). This standard PLP-dependent (3 replacement (Chapter 14) is catalyzed by cysteine synthase (O-acetylserine sulfhydrase).446 447 A similar enzyme is used by some cells to introduce sulfide ion directly into homocysteine, via either O-succinyl homoserine or O-acetyl homoserine (Fig. 24-13). In E. coli cysteine can be converted to methionine, as outlined in Eq. lb-22 and as indicated on the right side of Fig. 24-13 by the green arrows. In animals the converse process, the conversion of methionine to cysteine (gray arrows in Fig. 24-13, also Fig. 24-16), is important. Animals are unable to incorporate sulfide directly into cysteine, and this amino acid must be either provided in the diet or formed from dietary methionine. The latter process is limited, and cysteine is an essential dietary constituent for infants. The formation of cysteine from methionine occurs via the same transsulfuration pathway as in methionine synthesis in autotrophic organisms. However, the latter use cystathionine y-synthase and P-lyase while cysteine synthesis in animals uses cystathionine P-synthase and y-lyase. 

In addition to being a precursor of methionine in the activated methyl cycle, homocysteine is an intermediate in the synthesis of cysteine. Serine and homocysteine condense to form cystathionine. This reaction is catalyzed hy cystathionine -synthase. Cystathionine is then deaminated and cleaved to cysteine and a-ketohutyrate hy cystathioninase. Both of these enzymes utilize PLP and are homologous to aspartate aminotransferase. The net reaction is... 

B. Homocystine is produced when two homocysteine molecules form a disulfide bond. Homocysteine is produced when S -adenosylhomocysteine (SAH) releases adenosine. It is used in the synthesis of cystathionine. Homocysteine accepts a methyl group (from FH4 via vitamin B12) to form methionine. If the enzyme that cleaves adenosine from SAH is deficient, homocysteine levels will decrease. The other deficiencies would lead to increased levels of homocysteine. 

Cysteine synthesis is a primary component of sulfur metabolism. The carbon skeleton of cysteine is derived from serine (Figure 14.7). In animals the sulfhydryl group is transferred from methionine by way of the intermediate molecule homocysteine. (Plants and some bacteria obtain the sulfhydryl group by reduction of SOj to S2 as H2S. A few organisms use H2S directly from the environment.) Both enzymes involved in the conversion of serine to cysteine (cystathionine synthase and y-cystathionase) require pyridoxal phosphate. 

In those plants in which it has been determined, O-phosphohomoserine is present in concentrations of less than 25 ixM (Datko et al., 1974b, 1977). Since the for O-phosphohomoserine in threonine synthesis (in the absence of AdoMet) is approximately 2 mM (Madison and Thompson, 1976 Thoen et al., 1978), and for cystathionine synthesis approximately 7 mM (Madison and Thompson, 1976), it follows that (unless compartmentation is very extreme) each of these enzymes will be operating below its for O-phosphohomoserine, and the two enzymes will therefore compete for the available supply of this substrate. 

Transsulfuration is facilitated by the action of two vitamin Be-dependent enzymes, cystalhionine-p-synthase (CBS), the enzyme deficient in homocystinuria, and cystathionine-Y-lyase (CTH). CBS catalyzes the condensation of homocysteine and serine to cystathionine, and CTH subsequently catalyzes the hydrolysis of cystathionine to cysteine and a-ketobutyrale. Cysteine is important in protein synthesis and taurine synthesis and is a precursor to glutathione, a strong antioxidant and essential compound in detoxification of many xenobiotics [8,10,11]. 

E. colt and with yeast in isotopic experiments, and in enzymic experiments. The sjmthesis of methionine is the reverse of the synthesis of cysteine described previously (p. 243), via homocysteine and with cystathionine as... 

Nonoxidatioe. A single enzyme which catalyzes the deamination of L-serine and the synthesis of cystathionine has been partially purified from rat liver (496). A camino acid dehydrogena.se, distinct from alcohol dehydrogenase, has been obtained from N. crassa (497). 

Synthesis of cystathionine [Reaction (4)] and homocysteine [Reaction (5)J has been dembnstrated in extracts of a wide range of plants (Datko et al., 1974, 1977). The two activities have not been purified, and it is not known whether they are properties of a single enzyme as in . typhimurium, or of two separate enzymes, as in N. crassa (Flavin, 1975). 

Fig. 17.5 Effect of nitric oxide on the synthesis of methionine and S-adenosylmethionine and methylation reactions. NO inhibits methyltetrahydrofolate reductase (MTR). This results in a decrease in tetrahydrofolate (FH4) and methionine. Additional reduction in the FH4 level may occur by the NO-induced oxidation of ferritin, a compound that inhibits the proteasomal degradation of FH4. NO affects SAM synthesis not only by inducing a decrease in methionine synthesis but also by directly inhibiting the liver-specific methyl-thioadenosyltransferase I/III (MATI/III) isozymes. The fall in SAM level cannot be fully compensated by an increase in the extrahepatic isozyme MATH, since this enzyme is inhibited by its reaction product. The reduction in homocysteine utilization for methionine synthesis may result in homocysteine accumulation. This probably does not lead to a consistent rise in cystathionine and reduced glutathione synthesis, dne to a reduced stabilization of cystathionine P-synthase (CBS) by SAM. Consequently, an inciea.se in SAH, associated with a decrease in the SAM/SAH ratio, inhibits methyltransferases (MT) and DNA methylation. The reduction in SAM level may decrease IicBa activation, thus favoring NF-kB activity...

The pathway of methionine synthesis and the properties of the enzymes involved were reviewed previously in this series by Giovanelli et al. (1980). The synthesis of methionine is summarized in Fig. 5. It shows that cysteine reacts with the C4 skeleton of phosphohomoserine to form cystathionine and that the original C3 skeleton of cysteine is then removed, thus effecting the transfer of the thiol group of cysteine to homoserine to form homocysteine. This route,... 

Fig. 5. Regulation of the enzymes of methionine biosynthesis and related pathways. Enzymes catalyzing the synthesis of methionine and 5 -adenosylmethionine (SAM) from cysteine are (1) cystathionine y-synthase, (2) j9-cystathionase, (3) methionine synthase, and (4) SAM synthetase. Enzymes associated with the synthesis and metabolism of phospbohomoserine which are relevant to the regulation of methionine synthesis are (5) aspartate kinase, (6) homoserine kinase, and (7)...

A few of the enzymes of methionine metabolism have been studied further since the review by Giovanelli et al. (1980). Cystathionine y-synthase has still not been purified to establish whether a single protein supports both the synthesis of cystathionine from phospbohomoserine and cysteine (cystathionine y-synthase) and the synthesis of homocysteine from phospbohomoserine and... 

Phosphohomoserine is a substrate for both threonine synthase and cystathionine y-synthase. Thus, although threonine synthase is not involved in the synthesis of either methionine or phosphohomoserine the properties of this enzyme are relevant to methionine synthesis as it competes with cystathionine y-synthase for the same substrate. Moreover, as discussed in the ensuing section, the activity of threonine synthase and the synthesis of phosphohomoserine are regulated by products of the methionine biosynthetic pathway. 5-Aden-osylmethionine is an extremely potent positive effector of threonine synthase, virtually serving as an absolute requirement for enzyme activity (Aames, 1978 Giovanelli et a/., 1984 Madison and Thompson, 1976 Thoen eta/., 1978). In the presence of SAM, Giovanelli et al. (1984) found that threonine synthase had an extremely high affinity for phosphohomoserine (A = 2.2-6.9 nM). 

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